An acoustic well logging tool is generally cylindrically shaped and suitably sized for passage through a fluid filled well bore. Normally, the tool carries two or more transducers which are disposed and secured at a fixed distance from one another. In a typical acoustic tool having three transducers, one of the transducers serves as a transmitter of sound waves while the remaining transducers serve as receivers of sound waves. The receivers are spaced from one another at a predetermined distance and are disposed to one side of the transmitter along the longitudinal axis of the tool. In operation, the transmitter in the tool is electrically actuated periodically to emit pulses of acoustic energy (or pressure waves) which propagate outwardly from the transmitter with a velocity dependent upon the media traversed by the energy. The arrival of the acoustic energy at the successively positioned receivers is detected to trigger electrical circuits in the tool which function to ascertain a characteristic of the formation from the pulse of acoustic energy traveling the predetermined distance between the two receivers.
Acoustic energy as discussed above can be generated or intercepted by piezoelectric, magnetostrictive or other transducers in a well known manner.
In a typical well bore, an acoustic tool is commonly spaced from the wall of the well bore so that the emitted acoustic wave energy or pressure pulses are first omnidirectionally transmitted through fluid (usually mud) in the well bore and, after traveling through the fluid over the distance from the tool to the wall of the well bore, a portion of the traveling wave energy is transmitted to adjacent media surrounding the well bore. The characteristic velocity of wave motion or the wave energy through the fluids in the well is generally in the neighborhood of 5000 feet per second, while the characteristic velocity of wave motion through the adjacent media may vary from 5000 feet per second to 25,000 feet per second for compressional waves depending upon the type of media encountered. Other wave types have similar properties.
The portion of the acoustic wave energy transmitted into the media generally travels at a higher velocity than the corresponding portion of the wave energy traveling in the well bore fluid. Because of this, the portion of the wave energy traveling through media reaches a receiver prior to the time that the portion of the acoustic wave energy traveling through the fluids does. It is this feature of higher media velocity which permits measurement of the velocity of acoustic energy in the media surrounding a well bore.
Typically, each pulse of acoustic energy upon intercepting a receiver transducer generates an electrical signal containing a number of undulations, cycles or vibrations. The parameter measurement is generally based upon the detection of a given portion or characteristic of an electrical signal developed at the respective receivers for a given traveling pulse of acoustic energy. A commonly used characteristic of a corresponding electrical signal for detecting purposes, for example, is a voltage amplitude value. This is made possible because the undulations, cycles or vibrations of a typical electrical signal as developed from a typical pulse of acoustic energy generally include, in the first cycle, a first peak of a given polarity followed by a second peak of an opposite polarity and approximately three times the magnitude of the first peak and, in the second cycle, a third peak with a polarity similar to the first peak and about ten times the magnitude of the first peak. Hence, when a selected characteristic voltage amplitude value is exceeded, a detection signal for operating the electrical circuits can be developed.
The characteristic voltage amplitude value selected for detection purpose is generally such that detection will occur during the first cycle of a signal. The selection of a voltage amplitude characteristic of a first cycle of the signal to detect the first arrival of the acoustic signal is desirable because the voltage amplitude values of subsequent cycles are generally distorted due to acoustic reflections in the borehole.
In other more recent developments, acoustic tools have been designed to measure and record high fidelity full acoustic waveforms, including compressional, shear and Stoneley waves from the first cycle for a preselected time. This type of full waveform logging demands even greater precision in the measurement circuits and greater isolation of the desired acoustic signal from sources of extraneous noise.
From the foregoing discussion, it is apparent that a suitable supporting means for the transducers must be capable of preventing passage of detectable acoustic energy longitudinally between the transducers at a velocity higher than that of the adjacent media surrounding the well bore. Obviously, if the supporting means are not so constructed, the receiver circuit would be triggered prematurely by the acoustic energy traveling through the support means thereby preventing the electrical circuit from obtaining a parameter measurement accurately related to the velocity of the adjacent media.
Additionally, the newer demands for acoustic logging tools for the receiving and recording of entire sonic waveforms during a logging run, which may be provided for each of a series (or array) of acoustic receivers on the acoustic logging tool, require that not only must the housing provide a desired acoustic delay time between the transmitter and receivers but the housing must also minimize any undesirable secondary acoustic energy (noise) produced by the housing about the receivers.